Planning to tell your children about the birds and the bees? Think twice. Queen bees are among the most wanton of animals, mating with up to 20 males on their wedding flight. In tomorrow's Nature, Swiss researchers claim to know why: The queen's promiscuity protects the bee colony from parasitic plagues.

Parasites and their hosts are engaged in a continuous evolutionary arms race. Each time a parasite evolves a new trick, the victims have to adapt by acquiring a new defense. Experts think this may be the reason why organisms have sex in the first place: If all your offspring are clones, all could be wiped out by a single disease. Mix your genes with those of a partner, and the genetic jumble will produce a hodgepodge of offspring, at least some of which may survive.

Similarly, mating with lots of other partners may be even better than putting all your eggs in one paternal basket, ecologists Paul Schmid-Hempel and Boris Baer of the Federal Institute of Technology in Zürich reasoned. To test this hypothesis, they artificially inseminated 19 queen bumblebees. Twelve received sperm from just a single drone, so their daughters would be full sisters. Seven others were treated to a four-male sperm cocktail, producing mixed broods of sisters and half-sisters. The researchers then let the queens found colonies in a meadow near Basel and kept track of how these fared.

The one-father colonies suffered from parasitism much more than the multifather ones. One parasite in particular, a single-celled creature called Nosema, was almost absent from the latter, while every bee in the one-father colonies carried at least one of them. Moreover, the multifather broods, being healthier, were also twice as prolific. The researchers are not sure how genetic diversity increases the colony's health; they think parasites simply may have a smaller chance of spreading. But whatever the mechanism, they say, the queen's promiscuity is probably an evolutionary adaptation to increase genetic diversity and prevent disease in the colony.

"It's a stunning result," says Curtis Lively, an evolutionary biologist who studies the benefits of sex at Indiana University, Bloomington, and "a clever experimental test of an evolutionary hypothesis."